There are a variety of circumstances where stray light is found to be propagating within a cladding layer of an optical fiber. For example, a fusion splice between fibers of dissimilar core diameter (or mis-aligned core regions) may result in coupling a portion of a propagating optical signal into the nearby cladding layer. This unwanted coupling at a fusion splice is even more likely to occur when splicing a fiber bundle (or a tapered fiber bundle) to an output fiber. In any case, when this stray light is associated with a relatively high power level (for example, greater than 1 W or so), the light will begin to heat the fiber, resulting in thermal damage (and eventually system failure).
In particular, fiber-based lasers and optical amplifiers exemplify high power optical systems that are susceptible to this type of thermal damage. These high power, fiber-based components typically utilize a cladding layer of the fiber structure to introduce the requisite pump light to a section of rare earth-doped gain fiber. The gain fiber comprises a core region surrounded by at least two separate cladding layers, with the pump light introduced into the cladding layer adjacent to the core region.
At the far-end termination of the gain fiber, some residual pump light, as well as other unwanted spurious signals (for example, low NA signal light) may still be propagating along the cladding layer (referred to hereinafter at times as “cladding modes”). One existing method of removing the low NA signal light utilizes a long length of high index, re-coated double clad fiber. However, this high index coating is itself prone to thermal damage.
Thus, alternative non-coating based methods of stripping out low NA light (e.g., NA<0.15) are of interest.